When refreshing a frameset that includes an already POST submitted Wicket form in a frame, using the redirect to render strategy, Safari erroneously requests the form's original target by GET, rather than the location that was eventually redirected to. Therefore none of the form values are available in the request object and NPEs will occur trying to access them in places like AbstractConverter.java:55.
Because Form allows for a particular request method to be specified, I think it should also confirm that the expected method was used instead of waiting for an NPE in validation. The outcome is the same, but the cause of the error (the client) would be more evident in server logs, etc. Patch to come...